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P506: MEG is influenced by skull defects
Abstracts of Poster Presentations / Clinical Neurophysiology 125, Supplement 1 (2014) S1–S339
terstimulus intervals (ISIs) from a sample of patients undergoing DCS MEP monitoring and form recommendations based on the results. An ISI with minimal rheobase should be optimal as it would minimize the stimulus strength required to evoke responses; pulse duration (D) at the chronaxie should be optimal as it would minimize pulse energy (Prutchi and Norris, 2005). Methods: After identifying hand motor cortex, 5-pulse DCS thenar MEP rheobase and chronaxie were derived from current threshold measurements at 1, 0.5, 0.2, 0.1 and 0.05-ms D for 2, 3, 4 and 5-ms ISI in seven patients under propofol/opioid anesthesia. Results: Mean rheobase varied with ISI and was minimal with a 3-4 ms ISI (Fig. 1). Mean chronaxie was 0.18-0.19 ms with a 3-4 ms ISI (Fig. 2).
S185
Methods: A miniaturized electrical dipole was implanted in the brain of rabbits in vivo. Simultaneously, a 64-channel EEG and a 16-channel MEG were recorded first above intact skull and then above a skull defect. Skull defects were filled with agar gel of tissue-like conductivities. The dipole was moved beneath the skull defect and measurements were taken at regular steps. A computer tomography (0.4 mm3 voxels) provided the geometry of the defect and the position of the dipole. Results: The EEG amplitude increase reached factors of 2-10, while the MEG amplitude reduction reached -20% (Figs. 1 and 2). The EEG amplitude deviation is larger if the source is under the edge while the MEG amplitude deviation is larger if the source is centrally under the defect. The MEG topography change (RDM* =0.15) was geometrically related to the skull
Figure 1. Mean DCS thenar MEP rheobase (n=7) by interstimulus interval (ISI). Mean rheobase was significantly lower (P<0.05) at 3 or 4 than at 2 or 5-ms ISI. While mean values were slightly lower with a 4-ms than 3-ms ISI, the difference was not statistically significant; minimal rheobase occurred with a 4-ms ISI in four patients and with a 3-ms ISI in three.
Figure 2. Mean DCS MEP chronaxie (n=7) by interstimulus interval (ISI). These values were not significantly different except for between 2 and 4-ms ISI (p<0.05).
Conclusion: This report presents an approach to base DCS MEP stimulus parameters on rheobase and chronaxie. Preliminary results in seven of a planed n of twenty patients suggest that 3-4 ms ISI and 0.18-0.19 ms D might be generally optimal DCS MEP stimulus parameters. References: MacDonald DB, Skinner S, Shills J, Yingling C. Intraoperative motor evoked potential monitoring - A position statement by the American Society of Neurophysiological Monitoring. Clin Neurophysiol 2013;124(12):2291316. Prutchi D, Norris M. Stimulation of excitable tissues. In: Prutchi D, Norris M eds. Design and Development of Medical Electronic Instrumentation. John Wiley & Sons, 2005:305-368.
Poster session 30. MEG P506 MEG is influenced by skull defects
Figure 1. (A) EEG and MEG of a source at different locations relative to defect 1. Dipolar source shown as black bar with two poles, outline of the inner; middle and outer skull defect edges drawn in black; Sensor positions marked with grey dots; Minimum and maximum value and isoline difference displayed above each map. (B) Finite element model of the rabbit head showing the implanted source, skull defects, EEG and MEG.
S. Lau 1,2,3,4 , L. Flemming 3,5 , J. Haueisen 1,3 1 Technical University Ilmenau, Inst. for Biomedical Engineering and Informatics, Ilmenau, Germany; 2 University of Melbourne, Department of Medicine - St. Vincent’s Hospital, Melbourne, Germany; 3 University Hospital Jena, Biomagnetic Center, Department of Neurology, Jena, Germany; 4 University of Melbourne, NeuroEngineering Laboratory, Department of Electrical and Electronic Engineering, Melbourne, Germany; 5 Robert-KochHospital, Department of Traumatology and Orthopedics, Apolda, Germany Question: While the influence of skull defects on the electroencephalogram (EEG) has been reported, the magnetoencephalogram (MEG) had previously been hypothesized to have a negligible sensitivity to skull defects. The objective is to experimentally investigate the influence of conducting skull defects on the MEG and EEG.
Figure 2. MEG magnitude change (MAGrel) due to a skull defect for a series of locations of a tangential source beneath the defect in four animals (color-coded).
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Abstracts of Poster Presentations / Clinical Neurophysiology 125, Supplement 1 (2014) S1–S339
defect edge. The conductivity of the skull defect modulated the strength of change in the MEG & EEG. Dense spatial sampling revealed high spatial frequencies in MEG and EEG due to skull defects that are not detectable with current human helmet-type MEG devices and standard EEG setups. Conclusions: MEG and EEG changes due to a skull defect can be substantial and depend on the defect geometry and the relative orientation and position of the source. MEG forward modelling requires realistic volume conductor head model incorporating skull defects. This effect should be investigated further in humans.
P507 Predicting neurodevelopmental outcome of infants born <28 gestational weeks with magnetoencephalography and somatosensory evoked fields P. Nevalainen 1,2 , E. Pihko 3 , P. Rahkonen 4 , A. Lano 5 , S. Vanhatalo 1 , S. Andersson 4 , T. Autti 6 , L. Valanne 6 , M. Metsaeranta 4 , L. Lauronen 1,2 1 HUS Medical Imaging Center, Helsinki University Central Hospital, Dept. of Clinical Neurophysiology, Children’s Hospital, Helsinki, Finland; 2 HUS Medical Imaging Center, BioMag laboratory, Helsinki, Finland; 3 Aalto University School of Science, Brain Research Unit, O.V. Lounasmaa laboratory, Espoo, Finland; 4 Children’s Hospital, Helsinki University Central Hospital, Dept. of Neonatology, Helsinki, Finland; 5 Children’s Hospital, Helsinki University Central Hospital, Dept. of Child Neurology, Helsinki, Finland; 6 HUS Medical Imaging Center, Helsinki University Central Hospital, Helsinki, Finland Question: Despite great advances in neonatal intensive care, many extremely preterm infants still develop with neuromotor impairments. Neonatal neurological examination, serial cranial ultrasonography and term-age magnetic resonance imaging (MRI) are used to predict outcome, but fail to find some of the infants with neurodevelopmental impairments. We tested whether somatosensory evoked fields (SEFs) recorded with magnetoencephalography from the primary (SI) or secondary somatosensory areas (SII) could complement clinical and neuroimaging methods in predicting outcome of extremely preterm infants. Methods: We recorded SEFs to tactile stimulation of the index finger at term age in 39 infants born <28th gestational weeks and 46 fullterm control infants. Of the preterm infants 29 also underwent MRI, and neonatal neurological examination at term age, serial cranial ultrasonography in preterm era and clinical follow-up at 2-years corrected age. Results: Responses from SI were present in all infants without significant differences between preterm and fullterm infants. SII responses were absent significantly more often in preterm than fullterm infants (right hemisphere p=0.01; left hemisphere p=0.04). The preterm infants with absent SII response in either hemisphere had significantly worse developmental quotients and locomotor subscales (Griffiths Mental Developmental Scales) at 2-years follow-up than preterm infants with SII responses present (p<0.01). Of the 7 infants with unilaterally absent SII response 5 developed with complex minor neurological dysfunction or CP. Of these five, two were missed by neuroimaging and neonatal neurological examination. Conclusions: Evaluating SII responses at term age complements neonatal neurological examination and neuroimaging in predicting neurodevelopmental outcome of extremely preterm infants.
P508 MEG aids in differentiating continuous epileptiform activity from EEG breach rhythm, a case report J. Vanhanen 1,2 , J. Wilenius 1,2 , E. Kirveskari 1,2 1 HUS Medical Imaging Center, Helsinki University Central Hospital, Department of Clinical Neurophysiology, Helsinki, Finland; 2 Institute of Clinical Medicine, Faculty of Medicine, University of Helsinki, Department of Neurological Sciences, Helsinki, Finland Question: Breach rhythm may be reminiscent of epileptiform activity in EEG: how to differentiate between them? Patient and methods: A 40 years old male was referred to hospital because of flu and dizziness. Due to mild confusional state and fever, encephalitis was suspected and an EEG recording was performed. The EEG showed mild general slowing and continuous epileptiform spiking in the right occipital area, interpreted as a focal status epilepticus. The patient received antiepileptic drugs and several follow-up EEGs were obtained, all of which showed a similar finding. The patient was known to have symptomatic epilepsy since childhood and
focal gliosis in the right occipital lobe, as verified by stereotactic biopsy in 2011. The biopsy had caused a circular bone defect with a diameter of 8 mm, visible in MRI. The symptoms (confusional state and fever) disappeared in a few days, and since there were no visuospatial symptoms, the EEG interpretation was doubted. Bone defect is known to cause breach rhythm in EEG, which can be reminiscent of epileptiform activity. In our patient, the continuous epileptiform activity seemed to arise from the site where the brain biopsy had been taken. Thus, misreading of EEG might have been a plausible explanation for the finding, since a completely symptom-free focal status epilepticus seemed unlikely. Magnetoencephalography (MEG) signal is practically unaffected by the intervening tissues. Therefore we performed a MEG recording to verify the EEG finding of epileptiform activity. Results: The MEG showed continuous epileptiform activity in the right occipital area, and with equivalent current dipole modeling the sources of this activity were localized in the right occipital lobe at the area of gliosis seen in MRI. This finding was in accordance with previous EEG findings, confirming the existence of real continuous epileptiform activity. Conclusions: MEG can be a useful tool in verifying epileptiform activity in cases with potential intermingling breach rhythm, which is helpful especially if the EEG finding is in conflict with the clinical picture.
P509 Genetic influence is still maintaining on cerebral language function in elderly monozygotic twin: a MEG study T. Araki 1 , M. Hirata 1,2 , T. Yanagisawa 1,2 , H. Sugata 2 , M. Onishi 1 , K. Omura 1 , C. Honda 1 , K. Hayakawa 1 , S. Yorifuji 1 1 Osaka University Graduate School of Medicine, Division of Health Sciences, Suita, Japan; 2 Osaka University Medical School, Neurosurgery, Suita, Japan Aim: A genetic influence has been suggested for many cerebral functions, but little is known about language function. To estimate the genetic influence on the cerebral function during a language task, we used MEG to investigate the similarity of the cerebral oscillatory changes in elderly monozygotic twins. Methods: This study evaluated 55 native Japanese speakers, including 19 monozygotic twin pairs. We measured the brain activity during a verb generation task using a 160-channnel whole-head MEG system. In the verb generation task, a Japanese semantic word was presented visually after the presentation of a fixation point. Subjects were instructed to silently read each presented word, then to generate one verb related to the word immediately after word presentation. We investigated the spatio-temporal distribution of task-related cerebral oscillatory changes (ERS/ERD) using adaptive spatial filtering and a group statistical analysis. The control period was defined as the time period between 200 and 0 ms before stimulus onset, and the periods of interest were defined as continuously moving 200 ms windows from 0 ms to 1600 ms after stimulus onset. The windows were moved in steps of 100 ms. After detecting the peak coordinates of the ERD at 25-50 Hz, we estimated the power of the ERD in the peak coordinates using a time-frequency analysis. To compare the similarities in the power of the ERD within twin pairs, the correlation coefficient between the powers of the members of each twin pairs was determined. To compare twin pairs to unrelated subjects, we determined the correlation coefficient for 19 pairs of randomly drawn unrelated subjects. Results: The peak of the ERD at 25-50 Hz was estimated in the left frontal area based on the group statistical analysis. The ERD first appeared in the 200-400 ms post-stimulus window and was sustained until 1600-1800 ms. In the peak coordinate, the power of the ERD showed a high correlation among twin pairs. The correlation coefficient was 0.54 in the 800-1000 ms period at the maximum. On the other hand, unrelated subjects had a lower correlation in all time windows. Conclusion: We found that there was high similarity of the ERD of the left frontal area in elderly monozygotic twins. This finding suggested that the cerebral activity in left frontal area during a language task is under genetic influence.
terstimulus intervals (ISIs) from a sample of patients undergoing DCS MEP monitoring and form recommendations based on the results. An ISI with minimal rheobase should be optimal as it would minimize the stimulus strength required to evoke responses; pulse duration (D) at the chronaxie should be optimal as it would minimize pulse energy (Prutchi and Norris, 2005). Methods: After identifying hand motor cortex, 5-pulse DCS thenar MEP rheobase and chronaxie were derived from current threshold measurements at 1, 0.5, 0.2, 0.1 and 0.05-ms D for 2, 3, 4 and 5-ms ISI in seven patients under propofol/opioid anesthesia. Results: Mean rheobase varied with ISI and was minimal with a 3-4 ms ISI (Fig. 1). Mean chronaxie was 0.18-0.19 ms with a 3-4 ms ISI (Fig. 2).
S185
Methods: A miniaturized electrical dipole was implanted in the brain of rabbits in vivo. Simultaneously, a 64-channel EEG and a 16-channel MEG were recorded first above intact skull and then above a skull defect. Skull defects were filled with agar gel of tissue-like conductivities. The dipole was moved beneath the skull defect and measurements were taken at regular steps. A computer tomography (0.4 mm3 voxels) provided the geometry of the defect and the position of the dipole. Results: The EEG amplitude increase reached factors of 2-10, while the MEG amplitude reduction reached -20% (Figs. 1 and 2). The EEG amplitude deviation is larger if the source is under the edge while the MEG amplitude deviation is larger if the source is centrally under the defect. The MEG topography change (RDM* =0.15) was geometrically related to the skull
Figure 1. Mean DCS thenar MEP rheobase (n=7) by interstimulus interval (ISI). Mean rheobase was significantly lower (P<0.05) at 3 or 4 than at 2 or 5-ms ISI. While mean values were slightly lower with a 4-ms than 3-ms ISI, the difference was not statistically significant; minimal rheobase occurred with a 4-ms ISI in four patients and with a 3-ms ISI in three.
Figure 2. Mean DCS MEP chronaxie (n=7) by interstimulus interval (ISI). These values were not significantly different except for between 2 and 4-ms ISI (p<0.05).
Conclusion: This report presents an approach to base DCS MEP stimulus parameters on rheobase and chronaxie. Preliminary results in seven of a planed n of twenty patients suggest that 3-4 ms ISI and 0.18-0.19 ms D might be generally optimal DCS MEP stimulus parameters. References: MacDonald DB, Skinner S, Shills J, Yingling C. Intraoperative motor evoked potential monitoring - A position statement by the American Society of Neurophysiological Monitoring. Clin Neurophysiol 2013;124(12):2291316. Prutchi D, Norris M. Stimulation of excitable tissues. In: Prutchi D, Norris M eds. Design and Development of Medical Electronic Instrumentation. John Wiley & Sons, 2005:305-368.
Poster session 30. MEG P506 MEG is influenced by skull defects
Figure 1. (A) EEG and MEG of a source at different locations relative to defect 1. Dipolar source shown as black bar with two poles, outline of the inner; middle and outer skull defect edges drawn in black; Sensor positions marked with grey dots; Minimum and maximum value and isoline difference displayed above each map. (B) Finite element model of the rabbit head showing the implanted source, skull defects, EEG and MEG.
S. Lau 1,2,3,4 , L. Flemming 3,5 , J. Haueisen 1,3 1 Technical University Ilmenau, Inst. for Biomedical Engineering and Informatics, Ilmenau, Germany; 2 University of Melbourne, Department of Medicine - St. Vincent’s Hospital, Melbourne, Germany; 3 University Hospital Jena, Biomagnetic Center, Department of Neurology, Jena, Germany; 4 University of Melbourne, NeuroEngineering Laboratory, Department of Electrical and Electronic Engineering, Melbourne, Germany; 5 Robert-KochHospital, Department of Traumatology and Orthopedics, Apolda, Germany Question: While the influence of skull defects on the electroencephalogram (EEG) has been reported, the magnetoencephalogram (MEG) had previously been hypothesized to have a negligible sensitivity to skull defects. The objective is to experimentally investigate the influence of conducting skull defects on the MEG and EEG.
Figure 2. MEG magnitude change (MAGrel) due to a skull defect for a series of locations of a tangential source beneath the defect in four animals (color-coded).
S186
Abstracts of Poster Presentations / Clinical Neurophysiology 125, Supplement 1 (2014) S1–S339
defect edge. The conductivity of the skull defect modulated the strength of change in the MEG & EEG. Dense spatial sampling revealed high spatial frequencies in MEG and EEG due to skull defects that are not detectable with current human helmet-type MEG devices and standard EEG setups. Conclusions: MEG and EEG changes due to a skull defect can be substantial and depend on the defect geometry and the relative orientation and position of the source. MEG forward modelling requires realistic volume conductor head model incorporating skull defects. This effect should be investigated further in humans.
P507 Predicting neurodevelopmental outcome of infants born <28 gestational weeks with magnetoencephalography and somatosensory evoked fields P. Nevalainen 1,2 , E. Pihko 3 , P. Rahkonen 4 , A. Lano 5 , S. Vanhatalo 1 , S. Andersson 4 , T. Autti 6 , L. Valanne 6 , M. Metsaeranta 4 , L. Lauronen 1,2 1 HUS Medical Imaging Center, Helsinki University Central Hospital, Dept. of Clinical Neurophysiology, Children’s Hospital, Helsinki, Finland; 2 HUS Medical Imaging Center, BioMag laboratory, Helsinki, Finland; 3 Aalto University School of Science, Brain Research Unit, O.V. Lounasmaa laboratory, Espoo, Finland; 4 Children’s Hospital, Helsinki University Central Hospital, Dept. of Neonatology, Helsinki, Finland; 5 Children’s Hospital, Helsinki University Central Hospital, Dept. of Child Neurology, Helsinki, Finland; 6 HUS Medical Imaging Center, Helsinki University Central Hospital, Helsinki, Finland Question: Despite great advances in neonatal intensive care, many extremely preterm infants still develop with neuromotor impairments. Neonatal neurological examination, serial cranial ultrasonography and term-age magnetic resonance imaging (MRI) are used to predict outcome, but fail to find some of the infants with neurodevelopmental impairments. We tested whether somatosensory evoked fields (SEFs) recorded with magnetoencephalography from the primary (SI) or secondary somatosensory areas (SII) could complement clinical and neuroimaging methods in predicting outcome of extremely preterm infants. Methods: We recorded SEFs to tactile stimulation of the index finger at term age in 39 infants born <28th gestational weeks and 46 fullterm control infants. Of the preterm infants 29 also underwent MRI, and neonatal neurological examination at term age, serial cranial ultrasonography in preterm era and clinical follow-up at 2-years corrected age. Results: Responses from SI were present in all infants without significant differences between preterm and fullterm infants. SII responses were absent significantly more often in preterm than fullterm infants (right hemisphere p=0.01; left hemisphere p=0.04). The preterm infants with absent SII response in either hemisphere had significantly worse developmental quotients and locomotor subscales (Griffiths Mental Developmental Scales) at 2-years follow-up than preterm infants with SII responses present (p<0.01). Of the 7 infants with unilaterally absent SII response 5 developed with complex minor neurological dysfunction or CP. Of these five, two were missed by neuroimaging and neonatal neurological examination. Conclusions: Evaluating SII responses at term age complements neonatal neurological examination and neuroimaging in predicting neurodevelopmental outcome of extremely preterm infants.
P508 MEG aids in differentiating continuous epileptiform activity from EEG breach rhythm, a case report J. Vanhanen 1,2 , J. Wilenius 1,2 , E. Kirveskari 1,2 1 HUS Medical Imaging Center, Helsinki University Central Hospital, Department of Clinical Neurophysiology, Helsinki, Finland; 2 Institute of Clinical Medicine, Faculty of Medicine, University of Helsinki, Department of Neurological Sciences, Helsinki, Finland Question: Breach rhythm may be reminiscent of epileptiform activity in EEG: how to differentiate between them? Patient and methods: A 40 years old male was referred to hospital because of flu and dizziness. Due to mild confusional state and fever, encephalitis was suspected and an EEG recording was performed. The EEG showed mild general slowing and continuous epileptiform spiking in the right occipital area, interpreted as a focal status epilepticus. The patient received antiepileptic drugs and several follow-up EEGs were obtained, all of which showed a similar finding. The patient was known to have symptomatic epilepsy since childhood and
focal gliosis in the right occipital lobe, as verified by stereotactic biopsy in 2011. The biopsy had caused a circular bone defect with a diameter of 8 mm, visible in MRI. The symptoms (confusional state and fever) disappeared in a few days, and since there were no visuospatial symptoms, the EEG interpretation was doubted. Bone defect is known to cause breach rhythm in EEG, which can be reminiscent of epileptiform activity. In our patient, the continuous epileptiform activity seemed to arise from the site where the brain biopsy had been taken. Thus, misreading of EEG might have been a plausible explanation for the finding, since a completely symptom-free focal status epilepticus seemed unlikely. Magnetoencephalography (MEG) signal is practically unaffected by the intervening tissues. Therefore we performed a MEG recording to verify the EEG finding of epileptiform activity. Results: The MEG showed continuous epileptiform activity in the right occipital area, and with equivalent current dipole modeling the sources of this activity were localized in the right occipital lobe at the area of gliosis seen in MRI. This finding was in accordance with previous EEG findings, confirming the existence of real continuous epileptiform activity. Conclusions: MEG can be a useful tool in verifying epileptiform activity in cases with potential intermingling breach rhythm, which is helpful especially if the EEG finding is in conflict with the clinical picture.
P509 Genetic influence is still maintaining on cerebral language function in elderly monozygotic twin: a MEG study T. Araki 1 , M. Hirata 1,2 , T. Yanagisawa 1,2 , H. Sugata 2 , M. Onishi 1 , K. Omura 1 , C. Honda 1 , K. Hayakawa 1 , S. Yorifuji 1 1 Osaka University Graduate School of Medicine, Division of Health Sciences, Suita, Japan; 2 Osaka University Medical School, Neurosurgery, Suita, Japan Aim: A genetic influence has been suggested for many cerebral functions, but little is known about language function. To estimate the genetic influence on the cerebral function during a language task, we used MEG to investigate the similarity of the cerebral oscillatory changes in elderly monozygotic twins. Methods: This study evaluated 55 native Japanese speakers, including 19 monozygotic twin pairs. We measured the brain activity during a verb generation task using a 160-channnel whole-head MEG system. In the verb generation task, a Japanese semantic word was presented visually after the presentation of a fixation point. Subjects were instructed to silently read each presented word, then to generate one verb related to the word immediately after word presentation. We investigated the spatio-temporal distribution of task-related cerebral oscillatory changes (ERS/ERD) using adaptive spatial filtering and a group statistical analysis. The control period was defined as the time period between 200 and 0 ms before stimulus onset, and the periods of interest were defined as continuously moving 200 ms windows from 0 ms to 1600 ms after stimulus onset. The windows were moved in steps of 100 ms. After detecting the peak coordinates of the ERD at 25-50 Hz, we estimated the power of the ERD in the peak coordinates using a time-frequency analysis. To compare the similarities in the power of the ERD within twin pairs, the correlation coefficient between the powers of the members of each twin pairs was determined. To compare twin pairs to unrelated subjects, we determined the correlation coefficient for 19 pairs of randomly drawn unrelated subjects. Results: The peak of the ERD at 25-50 Hz was estimated in the left frontal area based on the group statistical analysis. The ERD first appeared in the 200-400 ms post-stimulus window and was sustained until 1600-1800 ms. In the peak coordinate, the power of the ERD showed a high correlation among twin pairs. The correlation coefficient was 0.54 in the 800-1000 ms period at the maximum. On the other hand, unrelated subjects had a lower correlation in all time windows. Conclusion: We found that there was high similarity of the ERD of the left frontal area in elderly monozygotic twins. This finding suggested that the cerebral activity in left frontal area during a language task is under genetic influence.